If you are planning on creating your own ultrasonic range finder then this really nifty module might serve you right.

Specifications

• Sound Pressure Level (SPL): 117dB (0dB=0.2n bar)
• Sensitivity (SEN): –60dB (0dB 1V/u bar)
• Impedance: 1K Ohm
• Ringing: less than 1mS
• Capacitance: 2400 pF (+/- 20%)
• Operating Frequency: 38KHz to 42KHz
• Drive Voltage: 20V RMS maximum (60V peak to peak maximum @10% duty)
• Temperature Range: 40C to +65C (-40F to +150F)